1. Field of the Invention
The present invention relates to a method for forming an oxidation passive layer, to a fluid-contacting part, and to a fluid feed system. In greater detail, the present invention relates to a method for forming an oxidation passive layer having a layer chiefly comprising aluminum oxides on the surface of stainless steel, a method for forming an oxidation passive layer chiefly comprising titanium oxides on a titanium base alloy surface, stainless steel or titanium base alloy having such passive layers formed thereon, and a fluid-contacting part and fluid feed system having parts in contact with a fluid (gas or liquid) employing these stainless steel and titanium materials.
2. Description of the Related Art
Chromium oxide passive layers are highly resistant to corrosion by various semiconductor manufacturing process gases. Moreover, the outgassing properties thereof are extremely superior, allowing such layers to be employed in vacuum devices, reduced-pressure devices, and gas supply pipes which require a high degree of cleanliness. These chromium oxide passive layers may also be used in supply pipes for ultrapure water and the like.
Recently, the high oxidizing power of ozone has been used in various technologies, such as cleaning of silicon substrate, ashing, and low-temperature CVD oxidation to develop highly efficient and integrated device.
However, in ozone supply piping materials, fluorine resins such as PVDF or the like, which are commonly employed in wet systems, and SUS316 and the like, which is commonly employed in gas systems, are markedly corroded by ozone. This represents a source of contamination, so that it is impossible to use such materials. Furthermore, as the ozone concentration increases, even the oxidation of the chromium oxide passive layers described above from Cr2O3 to CrO3 occurs as a result of the oxidizing power of the ozone. Therefore, it becomes impossible to maintain a high state of cleanliness in the piping and the atmosphere and the like.
In light of the above circumstances, the present invention has as an object thereof to provide a method for forming an oxidation passive layer which is highly resistant to corrosion by strongly oxidizing substances such as ozone.
Furthermore, it is an object of the present invention to provide stainless steel and titanium base alloys which are strongly resistant to corrosion by fluids containing ozone, as well as to provide fluid-contacting parts, process apparatuses, and fluid feed systems and discharge systems employing these corrosion resistent materials.
In a method for forming oxidation passive layers of the present invention, a stainless steel surface containing Al in an amount within a range of 0.5 percent by weight to 7 percent by weight is heat treated at a temperature within a range of 300xc2x0 C. to 700xc2x0 C. in a mixed gas atmosphere of an inert gas and 500 ppb to 1 percent H2O gas, and thereby, an oxidation passive layer containing aluminum oxides is formed.
Furthermore, in another method for forming oxidation passive layers in accordance with the present invention, a stainless steel surface containing Al in an amount within a range of 0.5 percent by weight to 7 percent by weight is polished to a Rmax of 0.7 micrometers or less, and baked in an inert gas atmosphere, whereby moisture is removed from the surface of the stainless steel, and subjected to heat treatment at a temperature within a range of 300xc2x0 C. to 700xc2x0 C. in a mixed gas atmosphere of an inert gas and 500 ppb to 1 percent H2O gas, and thereby, an oxidation passive layer containing aluminum oxides is formed.
In another method for forming oxidation passive layers in accordance with the present invention, a stainless steel surface containing Al in an amount within a range of 0.5 percent by weight to 7 percent by weight is subjected to heat treatment at a temperature within a range of 300xc2x0 C. to 700xc2x0 C. in a mixed gas atmosphere of an inert gas and 1 ppm to 500 ppm of oxygen gas, and thereby, an oxidation passive layer containing aluminum oxides is formed.
In another method for forming oxidation passive layers in accordance with the present invention, a stainless steel surface containing Al in an amount within a range of 0.5 percent by weight to 7 percent by weight is polished to a Rmax of 0.7 micrometers or less, and then, baked in an inert gas atmosphere, whereby moisture is removed from the stainless steel surface, and then heat treatment is conducted at a temperature within a range of 300xc2x0 C. to 700xc2x0 C. in a mixed gas atmosphere of an inert gas and 1 ppm to 500 ppm of oxygen gas, and thereby, an oxidation passive layer containing aluminum oxides if formed.
In the present invention, it is preferable that hydrogen gas be added to the mixed gas in an amount of 10 percent or less.
In another method for forming oxidation passive layers in accordance with the present invention, a stainless steel surface containing Al in an amount within a range of 0.5 percent by weight to 7 percent by weight is heat treated at a temperature within a range of 20xc2x0 C. to 300xc2x0 C. in a mixed gas atmosphere containing oxygen gas and at least 100 ppm of ozone gas, and thereby, an oxidation passive layer containing aluminum oxides is formed.
In a further method for forming oxidation passive layers in accordance with the present invention, a stainless steel surface containing Al in an amount within a range of 0.5 percent by weight to 7 percent by weight is polished to a Rmax of 0.7 micrometers or less, and baked in an inert gas atmosphere, whereby moisture is removed from the stainless steel surface, and then this is subjected to heat treatment at a temperature within a range of 20xc2x0 C. to 300xc2x0 C. in a mixed gas atmosphere containing oxygen gas and at least 100 ppm of ozone gas, and thereby, an oxidation passive layer containing aluminum oxides is formed.
In a further embodiment, it is characteristic that nitrogen gas is added in an amount of 10 percent or less to the mixed gas containing ozone gas as described above.
In the methods for forming oxidation passive layers in accordance with the present invention, it is preferable that the amount of Al contained in the stainless steel be within a range of 3 percent by weight to 6 percent by weight.
Additionally, in yet another embodiment, it is characteristic that the oxidation passive layer chiefly comprises a mixed oxide layer of aluminum oxides and chromium oxides.
In another method for forming oxidation passive layers in accordance with the present invention, a titanium base alloy surface is heat treated at a temperature within a range of 300xc2x0 C. to 700xc2x0 C. in a mixed gas atmosphere of an inert gas and 500 ppb to 1 percent H2O gas, and thereby, an oxidation passive layer comprising titanium oxides is formed.
In a further method for forming oxidation passive layers in accordance with the present invention, a titanium base alloy surface is polished to a Rmax of 0.7 micrometers or less, and baked in an inert gas atmosphere, whereby moisture is removed from the titanium base alloy surface, and then heat treatment is conducted at a temperature within a range of 300xc2x0 C. to 700xc2x0 C. in a mixed gas atmosphere of an inert gas and 500 ppb to 1 percent H2O, and thereby, an oxidation passive layer comprising titanium oxides is formed.
In another method for forming oxidation passive layers in accordance with the present invention, a titanium base alloy surface is heat treated at temperature within a range of 300xc2x0 C. to 700xc2x0 C. in a mixed gas atmosphere of an inert gas and 1 ppm to 500 ppm of oxygen gas, and thereby, an oxidation passive layer comprising titanium oxides is formed.
In a further method for forming oxidation passive layers in accordance with the present invention, a titanium base alloy surface is polished to a Rmax of 0.7 micrometers or less, and baked in an inert gas atmosphere, whereby moisture is removed from the surface of the stainless steel, and subsequently, by heat treatment at a temperature within a range of 300xc2x0 C. to 700xc2x0 C. in a mixed gas atmosphere of an inert gas and 1 ppm to 500 ppm of oxygen gas, thereby an oxidation passive layer comprising titanium oxides is formed. In the above heat treatment, it is preferable that hydrogen gas be in an amount of 10 percent or less.
In another method for forming oxidation passive layers in accordance with the present invention, a titanium base alloy surface is subjected to heat treatment at temperature within a range of 20xc2x0 C. to 300xc2x0 C. in a mixed gas atmosphere of oxygen gas and 100 ppm or more of ozone gas, and thereby, an oxidation passive layer comprising titanium oxides is formed.
In another method for forming oxidation passive layers in accordance with the present invention, a titanium base alloy surface is polished to a Rmax of 0.7 micrometers or less and baked in an inert gas atmosphere, whereby moisture is removed from the titanium base alloy surface, and heat treatment is conducted at a temperature within a range of 20xc2x0 C. to 300xc2x0 C. in a mixed gas atmosphere of oxygen gas and 100 ppm or more ozone gas, thereby an oxidation passive layer comprising titanium oxides is formed. In a further embodiment, 10 percent or less of nitrogen gas is added to the mixed gas.
In the present invention, the titanium base alloy contains 99 percent by weight or more of Ti, or alternatively, contains 99 percent by weight or more of Ti, 0.05 percent by weight or less of Fe, 0.03 percent by weight or less of C, 0.03 percent by weight or less of Ni, 0.03 percent by weight or less of Cr, 0.005 percent by weight or less of H, 0.05 percent by weight or less of O, and 0.03 percent by weight or less of N.
The stainless steel of the present invention has an oxidation passive layer having a thickness of 3 nm or more and chiefly containing aluminum oxides at the outermost surface thereof. Alternatively, an oxidation passive layer having a thickness of 3 nm or more and chiefly comprising aluminum oxides at the outermost surface is formed on a surface polished to a Rmax of 0.7 micrometers or less.
The amount of Al contained in the stainless steel is preferably within a range of 0.5 percent by weight to 7 percent by weight, and more preferably within a range of 3 percent by weight to 6 percent by weight.
The passive layer comprises mixed oxides of, chiefly, aluminum oxides and chromium oxides.
The titanium base alloy of the present invention has an oxidation passive layer having a thickness of 3 nm or more and comprising titanium oxides at the outermost surface formed thereon. Alternatively, an oxidation passive layer having a thickness of 3 nm or more and comprising titanium oxides at the outermost surface thereof is formed on a surface polished to a Rmax of 0.7 micrometers or less.
The titanium base alloy contains 99 percent or more of Ti, or alternatively, contains 99 percent or more of Ti, 0.05 percent by weight or less of Fe, 0.03 percent by weight or less of C, 0.03 percent by weight or less of Ni, 0.03 percent by weight or less of Cr, 0.005 percent by weight or less of H, 0.05 percent by weight or less of O, and 0.03 percent by weight or less of N.
The fluid-contacting part of the present invention has parts in contact with fluid which comprises stainless steel or titanium base alloy in accordance with the present invention.
The process apparatus of the present invention has parts in contact with fluid which comprise stainless steel or titanium base alloy in accordance with the present invention.
The fluid feed system of the present invention has parts in contact with fluid which comprise stainless steel or titanium base alloy in accordance with the present invention.
The fluid feed system of the present invention has parts in contact with fluid which comprise stainless steel or titanium base alloy in accordance with the present invention.
The discharge system of the present invention has parts in contact with fluid which comprise stainless steel or titanium base alloy in accordance with the present invention.
A method for forming oxidation passive layers on stainless steel will be explained as an example of a method for forming oxidation passive layers in accordance with the present invention.
The stainless steel employed contains 0.5 percent by weight to 7 percent by weight of Al. At amounts of less than 0.5 percent, a passive layer having high corrosion resistance cannot be formed, and if the amount is in excess of 7 percent, intermetallic oxides are formed and it is impossible to obtain a stable passive layer. It is particularly preferable that the amount of Al contained be within a range of 3 percent by weight to 6 percent by weight, and in this range, the aluminum oxide component ratio is further increased, and it is possible to form an oxidation passive layer having superior corrosion resistance with respect to ozone.
It is preferable that the surface of the stainless steel be polished so as to achieve a surface roughness Rmax of 0.7 micrometers or less, using electropolishing, composite electropolishing, polishing with polishing granules, buff polishing, or the like. By making the surface smooth, it is possible to reduce the amount of emitted gas, to increase the adhesion, and to suppress the generation of particulate matter so as to form a minute oxide layer. When the surface roughness is reduced, it becomes more difficult to form an oxidation passive layer, so that the surface roughness may be determined along with the formation temperature, the atmospheric concentration, the time, and the like, in accordance with the desired layer thickness and layer characteristics.
The following first through third oxidation methods are encompassed in the oxidation method of the present invention.
First, the first method is one in which heat treatment (300xc2x0 to 700xc2x0 C.) is conducted in an inert gas atmosphere containing a trace amount of moisture (500 ppb to 1 percent).
In the present method, as the moisture concentration increases, there is a tendency for the passive layer formation speed to increase. If the amount of moisture is less than 500 ppb, it is difficult to form a passive layer having aluminum oxide as a chief component thereof. Furthermore, the layer formation rate is extremely slow, so that this is not suitable for practical application. On the other hand, if the amount of moisture is in excess of 1 percent, although this is also related to the generation temperature, it becomes difficult to form a minute passive layer having a high degree of resistance to ozone.
As the heat treatment temperature increases, the rate of layer formation increases. At temperatures lower than 300xc2x0 C., there is almost no formation of the passive layer and practical application is impossible. At temperatures in excess of 700xc2x0 C., surface irregularities are produced, and the resistance to ozone also decreases. Therefore, the heat treatment temperature is set within a range of 300xc2x0 C. to 700xc2x0 C.
It is preferable that hydrogen gas be added to the inert gas, as described above, in an amount of 10 percent or less, and particularly preferably in an amount of 3 percent to 10 percent. By adding the hydrogen gas, it is possible to reduce the proportion of iron oxide in the oxidation passivation layer, and to form a passive layer having greater resistance to ozone.
The second oxidation method is one in which heat treatment (300xc2x0 C. to 700xc2x0 C.) is conducted in an inert gas atmosphere containing a trace amount (1 ppm to 500 ppm) of oxygen.
As the oxygen concentration increases, there is a tendency for the rate of generation of the passive layer to increase, and in the same manner as in the first method described above, in order to efficiently obtain a passive layer having a high degree of resistance to ozone, it is necessary that the oxygen concentration be within a range of 1 ppm to 500 ppm. Furthermore, for the same reason as in the first method, it is preferable that hydrogen gas be added to the inert gas in an amount of 10 percent or less.
The third method is one in which treatment (20xc2x0 C. to 300xc2x0 C.) is conducted in the presence of oxygen containing at least 100 ppm of ozone.
In this method, it is possible to produce an oxidation passive layer at low temperatures, and moreover, it is possible to form an oxidation passive layer having high resistance to ozone. Oxygen gas containing 100 ppm or more of ozone may be obtained by subjecting pure oxygen gas or a gas containing oxygen gas to an electric discharge by means of silent discharge. In such a case, it is preferable that 10 percent or less of nitrogen gas (preferably within a range of 4 percent to 6 percent) be admixed.
If the treatment temperature is in excess of 300xc2x0 C., the ozone will break down and the iron oxide component will increase, and the resistance to ozone will decrease. Therefore, this temperature is set at 300xc2x0 C. or below. Furthermore, if the treatment temperature is reduced to approximately room temperature, the formation of the layer is dramatically slowed, so that it is preferable to set the ozone concentration at, for example, 7 percent.
In the first through third methods described above, prior to conducting oxidation treatment, it is preferable that the surface to be subjected to oxidation treatment be polished to a Rmax of 0.7 micrometers or less in advance, and that this then be subjected to baking in an inert gas atmosphere (preferably within a range of 200xc2x0 C. to 600xc2x0 C.). By means of this preprocessing, the cleanliness of the layer is increased, and the resistance to ozone is further increased.
Next, a method for forming oxidation passive layers on a titanium base alloy will be explained.
Fundamentally, this is identical to the case in which stainless steel is employed. In other words, heat treatment is conducted in an inert gas atmosphere containing a trace amount (500 ppb to 1 percent) of moisture or a trace amount (1 ppm to 500 ppm) of oxygen, and thereby, it is possible to form an oxidation passive layer having high resistance to ozone which has titanium oxides as the chief component thereof.
Ti occludes hydrogen gas and becomes brittle, so that normally Ti is not brought into contact with hydrogen. However, in the present invention, event if 10 percent or less of hydrogen is added when the Ti is oxidized, the titanium does not become brittle as a result of the hydrogen. On the contrary, a minute and strong passive layer is formed.
Furthermore, if treatment (at 20xc2x0 C. to 300xc2x0 C.) employing oxygen gas containing at least 100 ppm of ozone is carried out as well, it is possible to form an oxidation passive layer having high resistance to ozone which has titanium oxides as the chief component thereof.
Examples of inert gases which were preferably employed in the present invention include, for example, N2 gas, Ar gas, and the like.
The stainless steel of the present invention has a oxidation passive layer having a thickness of 3 nm or more chiefly comprising aluminum oxides at the outermost surface thereof. Stainless steel having an oxidation passive layer containing chiefly aluminum oxides of a thickness of 3 nm exhibits extremely high resistance to ozone. It is preferable that the oxidation passive layer containing chiefly aluminum oxides and having a thickness of 3 nm be formed on a stainless steel surface having a Rmax of 0.7 micrometers or less; the resistance to ozone corrosion of such stainless steel is further increased.
The stainless steel base material of the present invention contains Al in an amount within a range of 0.5 percent by weight to 7 percent by weight, and more preferably within a range of 3 percent by weight to 6 percent by weight. By employing such stainless steel, it is easily possible to form an oxidation passive layer having aluminum oxides as a chief component thereof and having a thickness of 3 nm or more.
The titanium base alloy of the present invention has an oxidation passive layer having a thickness of 3 nm or more and chiefly comprising titanium oxides at the outermost surface thereof. The titanium base alloy having a 3 nm oxidation passive layer containing chiefly titanium oxides exhibits extremely high resistance to corrosion with respect to ozone. It is preferable that the oxidation passive layer containing chiefly titanium oxides and having a thickness of 3 nm be formed on the surface of stainless steel having a Rmax of 0.7 micrometers or less, whereby, the resistance to corrosion by ozone of such stainless steel is further increased.
The titanium base alloy of the present invention preferably contains Ti in an amount of 99 percent by weight or greater. More preferably, this titanium base alloy contains the further impurities of Fe in an amount of 0.05 percent by weight or less, C in an amount of 0.03 percent by weight or less, Ni in an amount of 0.03 percent by weight or less, Cr in an amount of 0.03 percent by weight or less, H in an amount of 0.005 percent by weight or less, O in an amount of 0.05 percent by weight or less, and N in an among of 0.03 percent by weight or less. By employing such a titanium base alloy, it is easily possible to form an oxidation passive layer having titanium oxides as a chief component thereof and having a thickness of 3 nm or more.
The oxidation passive layer formed in accordance with the present embodiment described above has anti-corrosive properties with respect to corrosive gases such as hydrogen chloride gas or the like, and has outgassing characteristics, which are superior, similarly to those of chromium oxide passive layers. In addition, such layers are extremely stable even with respect to fluids containing strongly oxidizing substances such as ozone. Accordingly, the stainless steel or titanium base alloy of the present invention may be used in process apparatuses such as vacuum or depressurized apparatuses, which require a highly clean atmosphere; as fluid-contacting parts of various types of gas or ultrapure water supply pipe systems, such as valves, filters, junctions, and the like; and in discharge systems, such as fluid feed systems, pumps, and the like. In addition, the stainless steel or titanium base alloy may be appropriately employed even with respect to cases in which fluids are employed which contain ozone and the like. Furthermore, it is a simple matter to employ the stainless steel of the present invention as material for lines having a diameter of few micrometers, and furthermore, an oxidation passive layer may be formed on the surface thereof, so that use in gas filters and the like is particularly appropriate.